After celebrating the seasons of Advent and Christmas, the Church focuses on the public ministry of Jesus during the season of Ordinary Time.
Ordinary Time always begins immediately following the feast of the Baptism of the Lord.
In January, the Church celebrates the feast of Epiphany on January 6 and in many places in the world this feast is transferred to the nearest Sunday.
The Baptism of the Lord is then celebrated on the next available Sunday, still within the Christmas season (in some cases the Baptism of the Lord is celebrated on the Monday after Epiphany Sunday).
Ordinary Monday
This means that the first official day of Ordinary Time is "Monday of the First Week of Ordinary Time."
The precise date changes every year, but in most cases occurs between January 10 and January 14.
What's interesting is how the first Sunday in the season of Ordinary Time is the 2nd Sunday in Ordinary Time.
Ordinary Time then continues as usual, interrupted by the seasons of Lent and Easter, until the first Sunday of Advent.
